The present invention relates to a control device for a continuously variable transmission.
A conventional control device for a continuously variable transmission of the feedback control type is shown in FIG. 1. This control device comprises a means 12 for determining target reduction ratio based on various kinds of signals 10 representative of the operating state of a vehicle, a feedback control means 14 for generating a shift command signal carrying instructions to establish the target reduction ratio determined by the means 12 for determining target reduction ratio, a shift actuator 16 operable on the shift command signal, a continuously variable transmission 18 where a reduction ratio is controlled in response to actuation of the shift actuator 16, and a means 20 for detecting actual reduction ratio in the continuously variable transmission 18. The feedback control means 14 is constructed as follows. A difference between the target reduction ratio generated by the mans 12 for determining target reduction ratio and the actual reduction ratio generated by the means 20 for detecting actual reduction ratio is obtained by an arithmetic unit 14a, this difference is integrated at an integrator 14b and then this integrated value is multiplied with a predetermined integral control gain K1 at a multiplier 14c. On the other hand, this difference obtained at the arithmetic unit 14a is multiplied with a predetermined proportional control gain K2 at a multiplier 14d. The values obtained at the multipliers 14c and 14d are added to each other at an adder 14e and the result is output to the shift actuator 16.
The conventional control device for continuously variable transmission of this type poses a problem as follows, however. Usually, in a continuously variable V-belt transmission, a reduction ratio vs. operating position of a shift actuator characteristic exibits a non-linear relationship as shown in FIG. 2. The fact that a relationship of a reduction ratio with a pulley bias force (i.e., a hydraulic fluid pressure creating a pulley bias force) is non-linear explains the non-linear relationship of the reduction ratio with the operating position of the shift actuator that determines a hydralic fluid pressure creating the pulley bias force. Although it is not impossible to modify the relationship of the reduction ratio with the operating position of the shift actuator to an approximately linear relationship by using a shift control valve, which is operable on the shift actuator and determines the hydraulic fluid pressure creating the pulley bias force, having a non-linear input/output characteristic, the use of such a shift control valve poses problems in quality control in manufacturing the shift control valves of the non-linear characteristic. As mentioned above, since the relationship of the reduction ratio with the operating position of the shift actuator is not linear, a variation (this variation hereinafter being called as "open loop gain") in reduction ratio for a unit amount in operating movement of the shift actuator differs depending upon operating positions assumed by the shift actuator. For example, as shown in FIG. 2, variations (G1, G2) for a unit amount (.DELTA.x) in operating movement of the shift actuator becomes large as the operating position of the shift actuator shifts toward a large reduction ratio (i.e., G1 is not less than G2). Regardless of the fact that the open loop gain differs depending upon the operating positions assumed by the shift actuator in this manner, the conventional control device for continuously variable transmission has used unvariable for the control gains (i.e., the previously mentioned K1, K2) over the whole operating positions assumed by the shift actuator. Therefore, although the control gains are consistent with the open loop gain within a certain range in operating position of the shift actuator, good driving feel cannot be expected within the other range because the response of the feedback control is too slow or hunting takes place frequently.